With the increasing use of fiber optics for communications, a need has arisen for improved means for routing signals carried by optical fibers. Switching for these fibers has previously been achieved with optoelectronic devices. Optical fibers are coupled with circuitry that permits the switching to be done electrically, and then the electrical signals are converted back to optical signals for further transmission. The conversion processes introduce loss and distortion into the signals, and the data rate that can be handled by such switches is limited by the speed of the electronic circuitry.
Purely optical methods for fiber optic switching show promise of supporting higher data rates. One such optical method involves the use of devices based on spatial light modulator (SLM) designs, especially those having reflective elements. An SLM is an array of electronically addressable elements, each of which is capable of separate mechanical movement in response to an electrical input. In the case of reflective SLM's, incident light illuminates the surface of the SLM and is redirected in a predetermined direction by those reflective elements that are positioned to reflect the light in that direction.
Many SLM's are binary in the sense that each pixel element may have either of two states. The element may be off, which means that it delivers no light. Or, the element may be on, which means that it delivers light at a maximum intensity. Other SLM's have three states, in which light may be redirected to either of two positions. The action the SLM performs upon light is in large part dependent upon the optical system into which the SLM is incorporated.
Various SLM architectures have been developed, which include variations with respect to the type of reflective elements and the addressing circuit. Reflective element types include elastomer, membrane, and cantilever or torsion beam types. Addressing may be by e-beam input, optically, or by means of an integrated circuit. Cantilever and torsion beam architectures, in combination with integrated circuit addressing, are described in an article entitled "Deformable-Mirror Spatial Light Modulators", by Larry J. Hornbeck, published in Proc. SPIE 1150, pp. 86-102 (1990).
In switching applications, the SLM is used to selectively couple an input fiber to one or more output fibers. For example, in an "on/off" type switch, light from an incoming fiber may be focussed onto a mirror element, which is positioned so that it either does or does not reflect the light into an output fiber. Or, in a "one to n" routing switch, incoming light may be focussed on an array of mirror elements, which are positioned to selectively redirect the light along further paths.
A problem with using SLM's as optical fiber switches is that many existing designs are not easily and cheaply manufactured. An important consideration in many designs is how to couple light in one fiber of the switch to a mirror element of the SLM without interfering with light in the other fibers of the switch. To achieve this isolated coupling, some designs require the fibers to meet the mirror element at an angle. The designs call for a precise positioning and securing of the fibers, as well as individual alignment of corresponding lenses, during manufacture. It is also difficult to devise sufficiently compact and rigid structures to support all of these various components at an angle, particularly for routing-type switches. A need exists for an SLM-based optical switch that is more easily manufactured and that possesses good rigidity in order to withstand vibration and shock.